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Old 05-29-2013, 03:39 AM #1
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Default Linear driver question

I just took apart some lasers that were given to me. They were green lasers but didn't work anymore (due to the lenses falling off...).

Anyhow the drivers are good. They are linear drivers, but in testing them I found out one strange behavior.

I took off the diode and connected the DMM to the positive and negative out for the diode with no dummy load. I connected the driver to the PS and obviously the Voltage out of the PS was the same that I had on the diode.
But as I increased the voltage the current also increased, for example at 4.2 Volts I had about 220mA. and at 3.6 I had about 180mA, or something like that (cant remember the exact numbers at the moment).

Is this normal due to the fact that there is no load at the end of the driver?

Also for what diodes could this be used (I guess that a red and IR would be good)?


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Old 05-29-2013, 04:49 AM #2
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Default Re: Linear driver question

Not so much of the host as the issue. But a linear driver needs a lot of volts so you generall need a host capable of 2 3.7cells.

Correct you do not need a dummy load for a linear driver. Just power source, dmm and adjustable driver.
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Old 05-29-2013, 05:24 AM #3
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Default Re: Linear driver question

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Originally Posted by chipdouglas View Post
Not so much of the host as the issue. But a linear driver needs a lot of volts so you generall need a host capable of 2 3.7cells.

Correct you do not need a dummy load for a linear driver. Just power source, dmm and adjustable driver.
Well I didn't understand you.
Ok let me explain something I have missed, this drivers come from a pencil type of host powered by 2AAA batteries. They were green lasers... So as I know not many batteries are needed for a green laser other than one.

They are linear in the voltage as the voltage that comes out is the same that goes in,'BUT as I increase the voltage the current increases as well... Why?
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Old 05-29-2013, 05:37 AM #4
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Default Re: Linear driver question

You didn't say what ld you would use it for but for 445 and 405 and more that you have to have minimum 2 3.7v cells if you're going to use a linear driver. The diodes alone use over 5 volts and the driver uses another volt or 2.
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Old 05-29-2013, 05:46 AM #5
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Default Re: Linear driver question

Chip I appreciate your answers really but they don't tell much on the problem I am experiencing.
It is true I didn't say for what I was going to use the driver, I did say however that they deliver 220mA at 4,2v now even if that was double at 8.4 (not tested since I don't know if the driver can take it) that would be 440mA, could you please tell me what laser in the 445 works with such a low mA?

Anyway someone can please tell me if there is a mayor problem with the driver since in my ignorance if a drive has constant current it should have constant current always and not depending on the voltage otherwise its not constant current anymore.... Am I correct???
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Old 05-29-2013, 05:50 AM #6
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Default Re: Linear driver question

It will become constant current once you are above the input voltage required for the regulator to deliver this full current.

Constant current regulators are actually, in practice, variable voltage regulators. All they do is adjust their output voltage until the current read on the shunt resistor matches the setpoint.

So, for example, if you have a load that doesn't actually pull the full current until it reaches a certain voltage, you'll read a slow increase in current on the regulator. Eventually when the input voltage goes high enough for the regulator to operate correctly, it should stay at a constant current as you increase the voltage beyond that point. I would have thought 8.4V would have been well beyond that stage, however. What are you using to measure the current?

For example, say you have a 100 ohm resistor as a load. Say you want to deliver a constant 120mA into it. If you are supplying the voltage regulator with 6V, and assume a rather high dropout (or headroom voltage, if you like) of 1.5V, this means you regulator can only supply up to 4.5V on it's output. Now, using ohms law, you can calculate 4.5V into a 100 ohm load, you only get 45mA. To get 120mA into the 100 ohm resistor, you'd need the voltage to be at least 12V, so the regulator input would need to be 13.5V, for example. Once you've gone past that 13.5V, the regulator then realizes it's at it's set current, and won't let it increase any further.

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Old 05-29-2013, 01:08 PM #7
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Default Re: Linear driver question

Quote:
Originally Posted by Things View Post
It will become constant current once you are above the input voltage required for the regulator to deliver this full current.

Constant current regulators are actually, in practice, variable voltage regulators. All they do is adjust their output voltage until the current read on the shunt resistor matches the setpoint.

So, for example, if you have a load that doesn't actually pull the full current until it reaches a certain voltage, you'll read a slow increase in current on the regulator. Eventually when the input voltage goes high enough for the regulator to operate correctly, it should stay at a constant current as you increase the voltage beyond that point. I would have thought 8.4V would have been well beyond that stage, however. What are you using to measure the current?

For example, say you have a 100 ohm resistor as a load. Say you want to deliver a constant 120mA into it. If you are supplying the voltage regulator with 6V, and assume a rather high dropout (or headroom voltage, if you like) of 1.5V, this means you regulator can only supply up to 4.5V on it's output. Now, using ohms law, you can calculate 4.5V into a 100 ohm load, you only get 45mA. To get 120mA into the 100 ohm resistor, you'd need the voltage to be at least 12V, so the regulator input would need to be 13.5V, for example. Once you've gone past that 13.5V, the regulator then realizes it's at it's set current, and won't let it increase any further.
Ok I understand this. I was using a DMM set in mA and shorted on the regulators output, no load.
I will have the parts to build my dummy this week. So I will be able to test it better.

Now a new question... ok to reach constant current you need a minimum voltage. And that is clear, but what happens when the voltage decrises, like when the battery drains? Than also the current will go down, that is normal?

Also since this drivers were installed in a host (factory installed) that used AAA batteries, and at that voltage they are NOT constant current, could this be the cause of such big fluctuations in some LPM readings in the green lasers?
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Old 05-30-2013, 04:21 AM #8
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Default Re: Linear driver question

A few of the cheap ones I've measured behave this way, too. It should hold the current constant from ~2.5-3.5V. Perhaps the regulator died and failed in a resistive way instead of open/short. Perhaps it was never designed properly to begin with.
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